Timing disconnect eth0 // connect wlan0

Hi everyone, I have a modem wifi with free unlimited nighttime data. I use my RasPi3 with Dietpi for Nextcloud and it works very well. I can not understand how to disconnect ethernet and connect wirelessly at a certain time to use the nightly wifi line which is much faster than my adsl. Does anyone know how to help me? Any guide is well accepted.
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